This invention pertains to magnetic bearing systems, and more particularly to a full levitation bearing systems utilizing passive radial magnetic bearings. The invention provides significantly increased radial load capacity without requiring additional magnets by reduction or elimination of unstable tilting moments generated by the individual passive permanent magnet bearings. In some cases, the invention permits stable levitation where it was not previously achievable and in other cases, the amount of magnet material required to achieve a desired radial bearing stiffness can be reduced, reducing size and costs.
Magnetic bearings are used in many applications for the benefits of long term, reliable operation, high-speed capability and low friction. Whenever possible, it is usually desirable to use passive radial magnetic bearings as opposed to active radial magnetic bearings because of much simpler, lower cost and more reliable construction. Passive radial magnetic bearings generate radial centering stiffness by the interaction of permanent magnets and sometimes with ferromagnetic poles. Because the centering force is provided passively, high frequency control electronics are not required. In a full levitation system, passive radial magnetic bearings can be located on opposite ends of a rotor and a single axial actively controlled magnetic bearing is all that is required for levitation.
Unfortunately, passive radial magnetic bearings usually provide only very low radial stiffnesses and load capacities. This characteristic property can hinder their implementation in some applications and can reduce performance when utilized in others. The low radial load capacity can require some devices that operate only vertically, so as to reduce the radial loads on the magnetic bearings, to have a small allowable tilt angle. To overcome the deficiencies of low radial stiffness and load capacity with passive radial magnetic bearings, designers have made the radial magnetic bearings larger, utilizing more magnet material to increase performance. Such solutions increase both the size and weight and also increase the cost of the magnetic bearings.
Unstable tilt moments occur in magnetic bearing systems because of a change in air gaps of the magnetic bearings when a tilt perturbation occurs. These unstable tilt moments are undesirable effects that are generated by individual passive magnetic bearings. When individual radial permanent magnet bearings are combined to provide a full levitation magnetic bearing system, the unstable tilt moments of individual bearings combine to reduce the radial stiffness and radial load capacity of the full system. The stable forces of other axes must overcome the unstable tilt moments to achieve a stable complete levitation. The radial stabilizing forces at one end of the rotor multiplied by the distance between the ends of the levitated rotor must be made larger than the unstable tilting moment of the opposite end. For this reason in disk type rotors, the unstable tilt moments can be very difficult to overcome due to the short distance between the passive radial magnetic bearings. Because the unstable tilt moment generated by a permanent magnet bearing is usually related to its diameter, one method for reduction of tilt moment forces is to make the magnetic bearings radially small in diameter. Unfortunately, this is not always possible because in many cases the desired large axial and/or radial forces require large magnetic bearing surface areas. The magnetic bearings could alternatively be distributed between multiple small diameter assemblies along the shafts. Unfortunately, this tends to generate unacceptably low flexural critical frequencies as well as large size and high costs.
One promising application for full levitation magnetic bearing systems utilizing passive radial magnetic bearings is in flywheel energy storage devices. These devices store and resupply energy as an alternative to conventional batteries, with the advantages of higher reliability, lower maintenance, known energy stored, temperature insensitivity and higher power capability. Flywheel energy storage devices include a high speed flywheel that rotates inside an evacuated container for elimination of aerodynamic drag. The flywheel has an attached motor/generator that accelerates and decelerates the flywheel for storing and retrieving energy. The flywheels are expected to rotate continuously at high speed for years, making the use of magnetic bearings very desirable. Because the flywheels rotate only to store energy, the bearing system need not have high radial loads applied. In many flywheel devices, the flywheels are made to rotate about a vertical axis, which further reduces radial bearing loading. However, because the flywheel can weigh as much as several hundred pounds for storing large amounts of energy, the bearings can still experience radial loading from the orientation axis being tilted slightly from vertical when the unit is installed.
The invention provides a full levitation magnetic bearing system with passive magnetic bearings that affords increased radial stiffness and load capacity. The magnetic bearing system uses two passive permanent magnet magnetic bearings, one each located on each side of the center of mass of the rotor to be levitated. An active axial magnetic bearing provides control for stable levitation. The magnetic bearing system provides increased radial stiffness and load capacity by the reduction or elimination of the unstable tilting moments generated by the individual passive radial permanent magnet magnetic bearings. Unstable tilting moments generated by passive magnetic bearings in full levitation magnetic bearing systems are reduced or eliminated by constructing the passive permanent magnet magnetic bearings so that they are concave toward the center of the rotor. The magnetic bearings are preferably curved or positioned piecewise on a curved surface with a radius approximately equal to the distance between the magnetic bearing and the central point about which tilt rotation would tend to occur. To maximize the radial stiffness and load capacity, the individual magnetic air gaps are preferably constructed to be perpendicular to the axis of rotation. Generation of detrimental unstable tilting moments is reduced or eliminated because, in this configuration, the magnetic bearing air gaps do not change significantly when the rotor is tilted with respect to the magnetic bearing stator portions. Hence, no change in the stored magnetic energy occurs due to tilting and no unstable moments are generated. Because the radial bearings at each end of the levitated body do not need to overcome unstable tilting moments from the opposite bearings, the fully levitated rotor system achieves increased radial stiffness and radial load capacity. The permanent magnet radial bearings can be made smaller, the tolerable tilt angle of installation for vertical axis systems can be increased and axially shorter as well as heavier rotors can also be stably levitated.